CodeMirror: Emacs bindings demo

The emacs keybindings are enabled by
including keymap/emacs.js and setting
the keyMap option to "emacs". Because
CodeMirror's internal API is quite different from Emacs, they are only
a loose approximation of actual emacs bindings, though.

Also note that a lot of browsers disallow certain keys from being
captured. For example, Chrome blocks both Ctrl-W and Ctrl-N, with the
result that idiomatic use of Emacs keys will constantly close your tab
or open a new window.